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By Damian Carrington, Baikonur The landing site for Beagle 2, launching on Monday aboard Mars Express, has been painstakingly picked to balance science with safety. The search for Mars’s water is the key mission of all three landers setting off for the Red Planet in June. The European Space Agency’s Mars Express will be swiftly followed by two NASA rovers. Water is crucial for life, and many scientists now believe once washed over the surface of Mars. Images from NASA’s Mars Odyssey revealed apparent river channels, lake beds, flood plains and sedimentary deposits. So the scientific imperative was to find sites where there is evidence of water action. But landing on Mars is a very tricky business. Ensuring a safe touchdown requires a flat site, with no ravine for the craft to tumble down, or peaks to crash into. It also needs to be at low altitude, so that the craft has as much of the thin Martian atmosphere as possible for braking. Other requirements are low winds and a position near the equator, so that the strength of the Sun will be sufficient to charge up solar powered batteries. For Beagle 2, the deliberations resulted in the choice of the Isidis Planitia, a large, flat basin just 10° north of the Martian equator. The evidence for water here comes in the form of numerous volcanic cones that resemble some on Earth formed by the explosive interaction of hot magma and water. The wide extent of the Isidis Planitia is also crucial, as it is not possible to precisely target Beagle 2. The area within which the lander is expected to end up is an ellipse 120 kilometres long and 40 km wide. This is a significantly narrower than early estimates, thanks to final confirmation of the craft’s mass (68.7 kg) and angle of entry (15 to 18°). The twin NASA rovers, due to arrive in January 2004, also have carefully selected landing sites. The first will target the Gusev crater, a 1600 metre deep hole gouged out of Mars by a meteorite impact. The fact that it lies at the end of the 900 km long Ma’adim Vallis channel and is unusually smooth has led scientists to suggest it was formerly a deep lake – an excellent place to look for signs of life. However, it is just 150 km across, meaning NASA’s aim will need to be spot on. The second quad-bike sized rover will land a few weeks later on the Meridiani Planum, a flat site at which images show layered rocks, perhaps laid down in water. There are also signs of haematite, an iron oxide mineral usually deposited in water on Earth. However, the signs of water at all three sites could have other explanations involving carbon dioxide or volcanism (New Scientist print edition 24 May 2003, p 23), which the landers will have to rule out before declaring proof of aqueous activity. The careful site selection process will be a major determinant of the scientific success of the Mars missions, and every predictable factor has been taken into consideration. But one phenomenon that cannot be forecast lurks in the background – giant dust storms. While rare, these events can shroud the planet in swirling red dust. This would obscure the Sun from reaching the landers’ solar panels, leaving them powerless. And even if a storm were spotted while the landers were on route, it would be too late to redirect the craft,